The MPLS standard, published under the auspices of the IETF (Internet Engineering Task Force) is a technique based on label switching which can be used to create a connection-oriented network from a datagram-type network such as the IP network. Detailed documentation of the MPLS protocol can be found on the Internet at www.ietf.org.
FIG. 1 diagrammatically shows an MPLS network 150 comprising a plurality of routers called LSR (label switching router) such as 100a, 100b, 110a, 110b, 110c and 120 interlinked by IP links. When an IP packet arrives at an ingress edge router 100a or 100b, called Ingress LSR, the latter assigns it a label according to its IP header and concatenates it with said packet. The router receiving the labeled packet replaces the label (incoming) with an outgoing label according to its routing table and the process is repeated from router to router as far as the egress edge router 120 (also called egress LSR) which deletes the label before transmitting the packet. Alternatively, the label deletion may already have been done by the penultimate router since the egress router 120 does not use the incoming label. An LSR router uses the label of the incoming packet (incoming label) to determine the output port and label of the outgoing packet (outgoing label). The path taken by a packet through the network from the ingress router 100a to the egress router 120 is called label-switched path (LSP). According to the example of FIG. 1 in which a path is represented by the arrows 105a, 105b and 105c, the LSR routers 110a, 110c crossed by the path and distinct from the ingress 100a and egress 120 edge routers are called transit routers. Also, the term “equivalence class” or “forward equivalence class” (FEC) is used to denote the set of IP packets that are transmitted along one and the same path.
The MPLS protocol makes it possible to force the IP packets to follow a pre-established LSP path which is not normally the optimal IP path in terms of number of hops or path metric. The technique for determining the path or paths to be taken is called traffic engineering or MPLS-TE (MPLS Traffic Engineering). The determination of the path takes account of the constraints on the available resources (constraint-based routing), particularly in terms of bandwidth on the various network links. Unlike the conventional IGP routing that works in a hop-by-hop mode (hop-by-hop routing), an LSP path is determined according to a so-called explicit mode (explicitly routed LSP or ER-LSP) wherein some or all of the nodes of the path from the ingress router to the egress router are determined. When all the nodes of the path are fixed, the routing can be called an “explicit routing” in the strict sense. A path determined by an explicit mode is also called MPLS tunnel.
Recommendation RFC 3564 entitled “Requirements for Support of Differentiated Services-aware MPLS Traffic Engineering”, hereinafter called DS-TE, makes it possible to set up MPLS tunnels guaranteeing a quality of service. A DS-TE tunnel that is unidirectional or bidirectional is set up between two edge routers along a path that observes a set of quality of service constraints such as bandwidth, service class and delay.
The determination of a tunnel is carried out centrally or in a distributed fashion. The centralized tunnel determination systems try to coordinate the placement of the tunnels in the label-switched communication network so as to optimize the use of the network resources. These systems are not well suited to use in a large scale network and are not very robust to abrupt changes in clients' needs.
Distributed tunnel determination systems try to place the tunnels in the network so as to react to changing clients' needs. The tunnel placement is normally performed by devices commonly called tunnel heads. Tunnel placements in distributed systems do not allow for a coordinated placement of the tunnels in the label-switched communication network and are often intensive network resource consumers.